A radio network is a network where network nodes transmit node signals which are received at one or more reception points in the radio network.
The network nodes, to exemplify, can be user equipments, UEs, which are transmitting node signals over the uplink in a 3GPP Long Term Evolution, LTE, network, in which case the reception points are connected to radio base stations, eNodeBs, of the radio access network. Thus, the node signals can be uplink signals in a radio access network.
The network nodes, to provide another example, can also be radio transceivers in a wireless local area network, WLAN, in which case the reception points are connected to access points of the WLAN. Thus, the node signals can be radio signals transmitted to a WLAN access point.
In both examples, a correct time alignment of node signals with respect to a reception time used for receiving the node signals is often crucial in order to attain reliable communication in the radio network. This time alignment of node signals can be calibrated either by properly adjusting the transmission time of node signals, or by adjusting, at the receiver side, the starting time for processing the one or more node signals. Irrespective of the method used, time alignment must often be re-calibrated continuously as network nodes move around in the radio network, since moving around generally changes the propagation delay of transmitted node signals with respect to a given reception point, and thus affects node signal time alignment.
In LTE radio access networks implementing orthogonal frequency division multiplexing, OFDM, as radio access technology, the time alignment between received OFDM symbols and the Fast Fourier Transform, FFT, time window used by an eNodeB in detecting said symbols, is important in order to be able to successfully decode the OFDM symbols. Also, a correct time alignment, with respect to said FFT time window, between two or more OFDM symbols received from different network nodes at a reception point, is important in order to maintain orthogonality between received node signals, i.e., in order to avoid the inter-carrier interference which may result if the time alignment is not sufficiently accurate.
Radio networks implementing OFDM as radio access technology in general, and LTE networks in particular, often implement two main mechanisms for achieving time alignment of uplink node signals and avoiding inter-carrier interference.
The first such mechanism is the use of cyclic prefixes which are added to the transmitted node signals in order to provide a measure of robustness against time alignment errors. The cyclic prefixes allow two or more OFDM symbols to be received as orthogonal node signals, i.e., not interfering significantly with each other, as long as the reception time window used for receiving the OFDM symbols starts during the cyclic prefix of all OFDM symbols. Hence, the cyclic prefixes serve to relax requirements on time alignment of node signals at a reception point, but they also introduce signaling overhead. Therefore, it is preferred to keep the cyclic prefix length as small as possible. In the single carrier, SC-, OFDM transmission format implemented for the uplink in LTE, the cyclic prefix is also used to account for any delay-spread of the physical radio channel due to, e.g., multi-path propagation.
The other time alignment mechanism is the timing advance, TA, commands which are transmitted in order to adjust transmission times of node signals to align different node signals at a given reception point with a reception window used for detecting node signals. The timing advance commands contain a negative offset in time between downlink and uplink subframes, and are used to account for propagation delay from a network node to a reception point. In this way all UEs transmitting in the uplink to a given reception point are time aligned to be received within the cyclic prefixes, thus assuring orthogonality between node signals.
Thus, in LTE, timing advance is done by the serving eNodeB with the aim of having a correct time alignment in the uplink for the serving eNodeB. However, if multiple reception points are used for uplink reception of a node signal, the timing advance mechanism does not necessarily time align node signals at all reception points. In fact, in some scenarios, differences in propagation delay or delay spread between a network node transmitting a node signal and the multiple reception points arranged to receive the node signal can be such as to make time alignment of the node signal in all reception points difficult or even impossible by use of timing advance commands and cyclic prefixes alone.
Consequently, there is a need for improved time alignment mechanisms for use in scenarios where multiple physically separated reception points are used for reception of one or more transmitted node signals.
Furthermore, in many scenarios where multi-point reception of node signals could potentially increase network performance, the inter-carrier interference level in the reception points can be too severe at times in order for the received node signal to be of any use. It is therefore not necessarily so that all reception points should be used to receive all node signals, and an informed selection of reception points therefore needs to be made.
Thus, there is also a need for a mechanism to select reception points to use in radio networks employing multiple reception points to receive a transmitted node signal.